The Pentac 50mm f/1.7 is a very nice lens. If you already own one, then there would be no point in getting a Pentax f/2.0. And unless you like the "wide-open look" (very shallow-depth-of-field), the the 50mm f/1.4 version offers no advantage either. I have both the f/1.7 and the f/1.4 versions in the M series, and I have not noticed any significant difference in image quality for the way I shoot. But I almost never use apertures wider than f/5.6. For all purpose walkabout shooting in daylight, I carry the f/1.7 version because it is a little lighter.
For the 35mm focal length, I have the f/2.0 version - which I use a lot. Again, I almost never shoot it wide open, but I do appreciate the brighter viewfinder compared to f/3.5 lenses. In between f/2.0 and f/3.5, Pentax makes the 35mm lens in a f/2.8 version which is probably a good compromise, but I have never used the f/2.8 version. I am guessing the 35mm/2.8 is going to be much easier to find - and less expensive - than the 35mm/2.0, so it is probably the more practical choice.